From Croft's Facebook:

Please note the last day of operation for the service 18 is the 21st September 2015

Thank you for your support in trying to make this service sustainable.

Regards James.

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And the cancellation is on VOSA today:

PB1065589/9 - BUSES 4 SALE LTD T/A CROFT COACH TRAVEL, CROFT COACH TRAVEL, UNIT 2 CROFTON ROAD, PORTRACK, STOCKTON-ON-TEES, TS18 2QZ

Cancellation Accepted: Operating between Middlesbrough Bus Station and Middlesbrough Bus Station given service number 18 effective from 21-Sep-2015.